The present invention relates to data communication systems. In particular, the invention relates to all areas where channel estimators and equalization methods are used in digital communication. The invention discloses methods and systems for detecting and rejecting a co-channel interferer. The invention is particularly suitable for mobile or base stations in cellular communication systems, but its application is not limited to them.
The cellular telephone industry has made phenomenal strides in commercial operations in the United States as well as the rest of the world. Growth in major metropolitan areas has far exceeded expectations and is rapidly outstripping system capacity. If this trend continues, the effects of this industry's growth will soon reach even the smallest markets. Innovative solutions are required to meet these increasing capacity needs as well as maintain high quality service and avoid raising prices.
In order to increase the capacity in modern Time Division Multiple Access (TDMA) cellular systems, such as Global System for Mobile Communication (GSM) or Enhanced Data rate for GSM Evolutions (EDGE), the cells are decreased in size. This reduction of size implies that the capacity will be interference limited and not sensitivity limited, because near co-channel interferers will dominate the noise figure (i.e., the amount of noise added in a received signal by the environment). Therefore, a significant portion of the noise received by a receiver will not be due to white noise. However, when designing a conventional receiver for a TDMA system, for instance in a mobile phone, the disturbance is often modeled as white noise. Thus, conventional receiver design is optimized for the sensitivity limited case and not for the interference limited case.
Referring to FIG. 1, a block diagram of a conventional TDMA receiver is shown. The received signal is down-converted, filtered, A/D converted and sampled at a symbol rate in the front end receiver (Fe RX) 10. The output from the Fe RX 10 can mathematically be written according to:                               y          t                =                                                            ∑                                  k                  =                  0                                L                            ⁢                                                           ⁢                                                h                  k                                ⁢                                  u                                      t                    -                    k                                                                        +                          e              t                                =                                                    H                T                            ⁢                              U                t                                      +                          e              t                                                          (        1        )            where H=[h0, . . . , hI]T is a complex-valued vector representing the radio channel, Ut=[ut, . . . , ut-L]T is a complex-valued vector representing the transmitted symbols (for instance 8-Phase Shift Keying (PSK)), and et is some kind of noise often assumed to be white. The received burst is fed to a synchronization and channel estimator unit (Channel Est.) 12 that correlates the received burst, containing a known symbol pattern (i.e., a training sequence) within the burst, with the known training sequence in order to find the synchronization position (i.e., the position within the received burst at which the training sequence starts). After the synchronization position is found, a channel estimation is performed in order to estimate the radio channel taps Ĥ. Standard estimation techniques, such as Least-Squares and the like, can be used to obtain the channel estimate. These techniques are well known in the art. The received burst, the estimated channel Ĥ and the synchronization position are then fed to an equalizer 14 that detects the transmitted symbols. Conceptually, the equalizer 14 tries to find the symbols minimizing the following:                                                         u              ^                        j                    =                      arg            ⁢                                                                       ⁢                                                                     ⁢                                          min                                  u                  ∈                  S                                            ⁢                                                           ⁢                                                ∑                                      t                    =                    1                                    N                                ⁢                                                                   ⁢                                                                                                                        y                        t                                            -                                                                        ∑                                                      k                            =                            0                                                    L                                                ⁢                                                                                                   ⁢                                                                                                            h                              ^                                                        k                                                    ⁢                                                                                    u                              ^                                                                                      t                              -                              k                                                                                                                                                                                2                                                                    ,                  j          =          1                ,        …        ⁢                                   ,        N                            (        2        )            where S represent the symbol constellation (i.e.,the set of symbols define for the given type of modulation, such as 8-PSK). The output from the equalizer is the decided hard symbol, the residuals given as:       ɛ    t    =            y      t        -                  ∑                  k          =          0                L            ⁢                           ⁢                                    h            ^                    k                ⁢                              u            ^                                t            -            k                              and a soft value, representing the uncertainty of the symbol estimate. The above described systems are well known in the art (see, J. Proakis, “Digital Communications”, McGraw-Hill Inc., New York, 1995).
The prior art systems do not provide for a co-channel interference-limited scenario (i.e., where the noise et is not white). Therefore, there is a need for an efficient method to detect and reject a co-channel interferer.